Cryogenic gas circulation and heat exchanger

ABSTRACT

A cryogenic system includes a housing of a cryogenic chamber, a cold source in the cryogenic chamber, and a gas circulation loop for circulating cryogenic gas between the cold source and material to be cooled in the cryogenic chamber. The circulation loop includes a gas pump, and a counter-flow heat exchanger connecting the gas pump to the cold source for cooling an in-flow of the cryogenic gas from the gas pump to the cold source with an out-flow of the cryogenic gas from the material to be cooled to the gas pump. In a preferred construction, the heat exchanger includes an outer tube and an inner tube nested within the outer tube, and a pair of three-port connector fittings attached to respective ends of the tubes.

RELATED APPLICATIONS

The present application claims the benefit of Nick Strickland and Stuart Wimbush U.S. Provisional Application Ser. 61/980,337 filed Apr. 16, 2014, incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to circulation of cryogenic gas in a cryogenic system, and a heat exchanger especially adapted for use in such a cryogenic system.

BACKGROUND

A cryogenic system often includes material to be cooled to a cryogenic temperature, a cryocooler for cooling the material to the cryogenic temperature, and a means for transporting heat from the material to the cryocooler. The means for transporting heat from the material to the cryocooler may include a gas circulation loop. For example, the material to be cooled to cryogenic temperature is a sample of material under analysis while properties of the material are measured as a function of temperature. In another example, the material to be kept at cryogenic temperature is a superconducting component of an apparatus that exploits the superconducting state of the material when the material is cooled to a cryogenic temperature. For example, the superconducting component is an electromagnet or a superconducting quantum interference device (SQUID) or a superconducting filter element or a superconducting electronic processing device (e.g. analogue/digital converter) or a rapid single flux quantum (RSFQ) computing element. A component of an apparatus can also be cooled to a cryogenic temperature because the performance of the component is improved at lower temperature. For example, the thermal noise of conventional electronic amplifiers and detectors is reduced at lower temperature, and the track resistance and therefore heat dissipation of conventional central processing units (CPUs) is decreased allowing higher clock speeds.

A material may also be cooled to a cryogenic temperature in order to process the material or to permanently change the properties of the material. For example, grinding of durable material to dust often becomes possible at a cryogenic temperature. Some materials are tempered when they are cooled to a cryogenic temperature. Some chemical reactions have high yield only at a cryogenic temperature.

The properties of common materials often change when the materials are cooled to a cryogenic temperature, and these changes complicate the design of cryogenic apparatus. These changes become substantial below a temperature of about 150 degrees Kelvin. Therefore, in this disclosure, “cryogenic” relates to a temperature below 150 degrees Kelvin. For example, “cryogenic gas” is a gas of a material that has a boiling point below 150 degrees Kelvin. Examples of cryogenic gas include helium, hydrogen, neon, nitrogen, fluorine, argon, oxygen, and krypton. The design of cryogenic apparatus for use below 70 degrees Kelvin is particularly difficult, because just a few cryogenic gases have boiling points below 70 degrees Kelvin. These gasses are helium (boiling at 4.2 degrees Kelvin), hydrogen (boiling at 20.3 degrees Kelvin), and neon (boiling at 27.1 degrees Kelvin).

Typically helium is used as a heat transfer fluid in a cryogenic system because helium has the lowest boiling point, enabling attainment of the lowest temperature, helium is inert and is not flammable in comparison to hydrogen, and helium is less expensive than neon. For example, a gas circulation loop in a cryogenic apparatus has used a cryogenic fan as a centrifugal pump to circulate helium gas between a cryocooler and the material to be cooled to a cryogenic temperature. A passive, gravity-assisted thermosiphon loop has also been used to circulate helium gas or liquid between a cryocooler and material to be cooled to a cryogenic temperature.

SUMMARY OF THE DISCLOSURE

The disclosed embodiments involve a counter-flow heat exchanger that enables a gas pump to circulate cryogenic gas between a cold source in the cryogenic chamber and material in the cryogenic chamber in order to cool the material to a cryogenic temperature. This permits the gas pump to operate at a temperature warmer than the temperature of the cold source and the material in the cryogenic chamber. This is a most elegant solution to the heat transfer problem where a high gas flow rate is required. A gas pump operating at a temperature warmer than a cryogenic temperature, and in particular in a room temperature environment outside of the cryogenic chamber, is an efficient solution to the heat transfer problem provided that a counter-flow heat exchanger is used to cool the inflow of warm cryogenic gas from the gas pump into the cryogenic chamber with the outflow of cold cryogenic gas from the cryogenic chamber to the gas pump.

In contrast to a cryogenic fan, a gas pump outside of the cryogenic chamber working in combination with a counter-flow heat exchanger has no moving parts that must be made to operate at cryogenic temperatures, and has no drive shaft or power coupling between a cryogenic component and the external environment. In contrast to a gravity-assisted thermosiphon, a gas circulation loop driven by a gas pump does not require a particular placement of the coldest and warmest elements of the loop such that denser cooled gas or liquid falls while warmed gas or liquid rises. This particular placement may not be convenient or possible in all applications and also may not provide the flow rate required.

In one embodiment, a cryogenic system includes a housing of a cryogenic chamber, a cold source in the cryogenic chamber, and a gas circulation loop for circulating cryogenic gas between the cold source and material to be cooled in the cryogenic chamber. The gas circulation loop includes a gas pump, and a counter-flow heat exchanger connecting the gas pump to the cold source for cooling an inlet flow of the cryogenic gas from the gas pump to the cold source with an outlet flow of the cryogenic gas from the material to be cooled in the cryogenic chamber to the gas pump. In a preferred embodiment, the gas pump operates in a room-temperature environment.

In another embodiment, a cryogenic system includes a housing of a cryogenic chamber, a staged cryocooler having a plurality of cold heads in the cryogenic chamber, and material to be cooled to a cryogenic temperature in the cryogenic chamber, and a gas circulation loop for circulating cryogenic gas between the cold heads of the cryocooler and the material in the cryogenic chamber. The gas circulation loop includes a gas pump, and a counter-flow heat exchanger connecting the gas pump to the cold heads for cooling an inlet flow of the cryogenic gas from the gas pump to the cold heads with an outlet flow of the cryogenic gas from the material to be cooled in the cryogenic chamber. The flow of cryogenic gas from the heat exchanger to the cold heads is progressively cooled by heat transfer to a higher-temperature one of the cold heads and then to a lower-temperature one of the cold heads before passing to the material to be cooled in the cryogenic chamber. In a preferred embodiment, the gas pump operates in a room-temperature environment.

In another aspect, the present disclosure describes a method of cooling a material in a cryogenic chamber. The method includes circulating cryogenic gas in a gas circulation loop between a cold source in the cryogenic chamber and the material in the cryogenic chamber. The gas circulation loop includes a gas pump and a counter-flow heat exchanger. The method further includes the gas pump pumping the cryogenic gas through the gas circulation loop, and the counter-flow heat exchanger cooling an inlet flow of the cryogenic gas from the gas pump to the cold source with an outlet flow of the cryogenic gas from the material in the cryogenic chamber to the gas pump. In a preferred embodiment, the gas pump operates in a room-temperature environment.

This disclosure also provides a counter-flow heat exchanger especially adapted for circulating cryogenic gas between a room-temperature environment and a cryogenic environment in a cryogenic chamber. The heat exchanger includes an outer tube and an inner tube nested within the outer tube, and a pair of three-port connector fittings attached to respective ends of the tubes. The three-port connector fittings provide a sealed environment with independent access to a first flow of fluid and a second flow of fluid through the heat exchanger, while preventing mixing of the first flow of fluid and the second flow of fluid. The first flow of fluid flows between the three-port connector fittings inside the outer tube and outside the inner tube, and the second flow of fluid flows between the three-port connector fittings inside the inner tube. In a preferred embodiment, the tubes are wound into a helix for compactness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a cryogenic system using a room-temperature pump to circulate cryogenic gas between components in a cryogenic chamber;

FIG. 2 is a schematic diagram of a more specific example of a cryogenic system using a room-temperature pump to circulate cryogenic gas between components in a cryogenic chamber;

FIG. 3 is perspective view of a helical counter-flow heat exchanger introduced in FIG. 2;

FIG. 4 is a lateral cross-section of a lower end of the heat exchanger of FIG. 3; and

FIG. 5 is a collection of graphs showing temperature as a function of time during operation of the cryogenic system of FIG. 2.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the present disclosure.

FIG. 1 shows a cryogenic system 20 including a housing 21 for a cryogenic chamber 22. The cryogenic chamber 22 contains material 23 to be cooled to a cryogenic temperature. The housing 21 provides a way of thermally insulating all cryogenic components from an external environment that is substantially above any cryogenic temperature. For example, the environment external to the housing 21 is a room-temperature environment, and the region inside the housing 21 is evacuated with a vacuum pump to reduce convective heat transfer from the external environment to the cryogenic components in the chamber 22. The housing 21 may also include layers of heat insulation to reduce radiative heat transfer from the external environment to the cryogenic components in the chamber 22, and components in the cryogenic chamber 22 may be wrapped with heat insulation in order to reduce radiative heat transfer to those components. For example, the heat insulation is super insulation including multiple layers of metalized plastic film.

In order to cool the material 23 to a cryogenic temperature, the cryogenic system 20 includes a cold source 25, which in this example is a cold head of a cryocooler 24. The cryocooler pumps heat from the cold head 25 to a heat sink 26 in order to reduce the temperature of the cold head below the cryogenic temperature to which the material 23 is to be cooled. The heat sink 26 expels the heat to the external environment. For example, the heat sink 26 is an air-cooled radiator, or the heat sink 26 is a heat exchanger cooled by a flow of tap water. In an alternative system, the cryocooler 24 could be replaced with another kind of apparatus providing a cold source in the cryogenic chamber. For example, the cold source could be a container of liquid nitrogen, and the liquid nitrogen would boil and nitrogen gas would be expelled to the environment outside of the housing 21 when heat would flow to the container of liquid nitrogen.

For transferring heat from the material 23 to the cold head 25, the cryogenic system 20 includes a gas circulation loop 27 circulating cryogenic gas between the material 23 and the cold head 25. For example, the gas circulation loop 27 includes a heat exchanger 28 fastened to the cold head 25. The cryogenic gas flows through the heat exchanger 28 and flows from the heat exchanger to the material 23 to cool the material by picking up heat from the material. For example, the cryogenic gas directly contacts the material 23 or contacts a container of the material or a heat exchanger attached to the material or a container of the material. In addition, the temperature of the material to be cooled could be controlled by attaching an externally-controlled heater to the material, or the container of the material, or the heat exchanger attached to the material or container.

The cryogenic gas circulation loop 27 includes a gas pump 29 providing a motive force for circulating the cryogenic gas through the loop. Although a cryogenic fan could be used as a gas pump, it has been discovered that a conventional gas pump 29 operating in a room-temperature environment can be used instead by inserting a counter-flow heat exchanger 30 between the gas pump 29 and the cryogenic components of the gas circulation loop 27 in the cryogenic chamber 22. The counter-flow heat exchanger 30 uses the cold out-flow of cryogenic gas to pre-cool the warm in-flow of gas before the in-flow of cryogenic gas reaches the cold head 25 of the cryocooler 24. In this manner, a significant pre-cooling (e.g., 200 degrees Kelvin or more) can be obtained, reducing the load on the cryocooler 24, and thereby enabling a lower base temperature to be reached.

The conventional gas pump 29 is a positive displacement pump so that it has a higher efficiency for pumping light cryogenic gas, such as hydrogen and helium, than a cryogenic fan. A conventional gas pump 29 is a common industrial component, so that it is less expensive than a cryogenic fan. However, these advantages of a common gas pump over a cryogenic fan are offset by some heat loss and pressure drop due to the counter-flow heat exchanger 30. Nevertheless, it has been found that if the counter-flow heat exchanger 30 is properly dimensioned for the flow rate of the cryogenic gas, then the combination of the conventional gas pump 29 and the counter-flow heat exchanger 30 is an acceptable substitute for a cryogenic fan.

FIG. 2 shows a more specific example of a cryogenic system 40. The cryogenic system 40 includes a housing 41 providing an evacuated cryogenic chamber 42 insulated from the external environment. In this example, material 43 to be cooled to a cryogenic temperature is inserted into a canister 44 sufficiently strong to be pressurized to slightly above atmospheric pressure within the vacuum of the cryogenic chamber 42. For example, in the system of FIG. 2, the material 43 to be cooled to a cryogenic temperature can be inserted by opening the top of the canister 44 without breaking the vacuum in the cryogenic chamber 42.

The cryogenic system 40 includes a two-stage cryocooler 45 having a first stage cold head 46 at a cryogenic temperature, a second stage cold head 47 at a colder temperature than the first stage cold head, and a heat sink 48 to the external environment. A cryogenic gas circulation loop 49 circulates cryogenic gas through a heat exchanger 50 fastened to the first stage cold head 46. From the heat exchanger 50, the cryogenic gas is circulated to a heat exchanger 51 fastened to the second stage cold head 47. The cryogenic gas flows through the heat exchanger 51 to a lower port 52 of the canister 44 and into the canister, so that the cryogenic gas comes into direct contact with the material 43 to be cooled to the cryogenic temperature. A heater in contact with the material 43 or in contact with the canister 44 or in the gas steam could be used to control the material temperature either directly or via the gas temperature.

The cryogenic gas then flows out an upper port 53 of the canister 44 and into a first passage of a counter-flow heat exchanger 54 disposed between the cryogenic environment of the cryogenic chamber 42 and an external room-temperature environment. From the first passage of the counter-flow heat exchanger 54, the cryogenic gas flows into a gas pump 55 in the room-temperature environment. From the gas pump 55, the cryogenic gas flows into a second passage of the counter-flow heat exchanger 54 leading back to the heat exchanger 50 on the first stage cold head 46. In this manner the gas flowing to the pump 55 is substantially warmed, allowing the pump to operate at ambient temperature, while the gas returning back to the first stage cold head 46 is substantially cooled to a cryogenic temperature.

In the example of FIG. 2, the counter-flow heat exchanger 54 includes a tubular section 56 wound into a helix and having two ends terminated with respective three-port T-connector fittings 57, 58. As further described below with reference to FIG. 4, the tubular section 56 consists of a pair of nested metal tubes having different diameters. The port of each T-connector fitting 57, 58 directly opposite from the respective end of the tubular section 56 provides a gas flow path to the first passage through the counter-flow heat exchanger 54. The port of each T-connector fitting 57-58 at the base of the “T” provides a gas flow path to the second passage through the counter-flow heat exchanger 54.

For charging of the cryogenic gas circulation loop 49 with cryogenic gas such as helium, a valve 59 is opened to admit the cryogenic gas into the loop through a T-connector fitting 60. Prior to admitting the cryogenic gas, the loop 49 is evacuated by opening a valve 61 to a vacuum pump 62. A purge line 63 connects the valve 61 to the canister 44. The purge line 63 is configured such that gas is exhausted directly from the canister 44 while the other gas lines in the gas circulation loop 49 are configured such that helium gas is admitted first through the cold head heat exchangers 50, 51 before entering the canister 44. In this manner any air entering the canister 44 during installation of materials in the canister does not enter through the cold head heat exchangers 50, 51 where it would freeze and cause a blockage.

FIG. 3 shows the helical counter-flow heat exchanger 54 in greater detail. In this example, the helix of the tubular section 56 includes ten turns. There is a substantially uniform gap between neighbouring turns to reduce heat transfer between the neighbouring turns.

FIG. 4 shows that the tubular section 56 of the heat exchanger 54 includes a pair of coaxial tubes including an outer tube 71 and an inner tube 72 nested within the outer tube 71. An annular region 73 between the tubes 71, 72 provides the first passage through the heat exchanger 54 (from the upper three-port T-connector fitting 58 to the lower three-port T-connector fitting 57 in FIG. 3), and the central region 74 of the inner tube 72 provides the second passage through the heat exchanger (from the lower three-port T-connector fitting 57 to the upper three-port T connector fitting 58 in FIG. 3). The three-port T-connector fittings 57, 58 provide a sealed environment with independent access to each of the nested tubes 71, 72 for counter-flow through the heat exchanger, while preventing any mixing of the two counter-flows. The three-port T-connector fittings independently seal the tubes 71, 72, while allowing attachment to the rest of the components in the gas circulation loop 49.

In a preferred arrangement, the internal diameters of the two tubes are chosen so that the cross-sectional areas of the two passages 73, 74 are approximately equal. For example, the outer tube 71 is 5/16 inch tubing, and the inner tube 72 is 3/16 inch tubing. The overall length of the tubes is chosen to be sufficient to adequately thermally decouple the cold end from the hot end and to provide adequate heat exchange between the gas streams. For example, the overall length of the tubes is about six feet. For compactness, the tubes are coiled into a helix having a 2.5 inch diameter, and the helix is about 3.5 inches high.

The outer tube 71 is preferably made of a low thermal conductivity material. For example, the outer tube 71 is a type 304 or 316 stainless steel and has an outer diameter of 5/16 inches and a wall thickness of 0.035 inches. The outer tube 71 provides mechanical strength through its thickness, in order to contain the cryogenic gas when the cryogenic chamber is evacuated and maintain the shape of the helical counter-flow heat exchanger 54. The inner tube 72 should be thermally conductive, and should have as thin a wall as possible while maintaining structural integrity so as to maximise heat transfer between the two passages 73, 74, and minimise heat transfer along the length of the tube. A suitable material for the inner tube 72 is copper. For example, the inner tube 72 is a standard copper tube having an outer diameter of 3/16 inches and a wall thickness of 0.028 inches. Higher-purity copper, such as electrolytic tough pitch (ETP) or oxygen free high conductivity (OFHC) copper, could be used to provide higher thermal conductivity especially at lower cryogenic temperatures.

Heat transfer between the counter-flows of cryogenic gas across the thin wall of the high thermal conductivity inner tube 72 allows an equalization of temperatures in the counter-flows at any given point along the length of the inner tube 72. At the same time, the overall length of the inner tube 72 and the outer tube 71 combined with the low thermal conductivity of the outer tube 71 and the thin wall of the inner tube 72 avoids the equalization of temperatures from the hot end to the cold end of the heat exchanger 54. In this manner, the out-flowing (cold) gas emerges from the hot end (the three-port connector fitting 58) of the heat exchanger 54 at or close to the temperature of the in-flowing (warm) gas. Similarly, the in-flowing (warm) gas emerges from the cold end (the three-port connector fitting 57) of the heat exchanger 54 close to the temperature of the out-flowing (cold) gas. The three-port connector fittings 57, 58 ensure that the counter-flows do not mix or leak into the cryogenic chamber or into the room-temperature external environment.

The minimum practical diameter of the helix is determined primarily by the minimum bend diameter of the outer tube. The minimum bend diameter of a tube is the minimum diameter of a bend that can be made by winding of the tube around a matching cylindrical grooved bender die without having the tube collapse. For example, the minimum bend diameter of a standard 5/16 inch steel or stainless steel tube is 1 and ⅞ inches.

The three-port T-connector fitting includes a central body 75 and three tubular arms 76, 77, 78. Each of the tubular arms 76, 77, 78 defines a respective port. Two of the arms 76, 77 are opposite arms of a “T”, and the other arm 78 is the base of the “T”. Originally a cylindrical bore of uniform diameter passed through the body 75 between the opposite arms of a T, and this bore intersected a bore 80 from the arm 78 at right angles. The inner tube 72 has an outer diameter matching the diameters of these bores. The original bore in the arm 76 is enlarged by drilling to just beyond the intersection of the “T” to provide the bore 79 in the arm 76 that receives the tubular section 56 of the heat exchanger 54. The bore 79 has a diameter equal to the inner diameter of the outer tube 71 to extend the passage 73 for flow of the cryogenic gas to the bore 80 and out the port of the arm 78. The second passage 74 extends all the way to the port of the arm 77 for the flow of the cryogenic gas in the port of the arm 77 and through the second passage 74 to the upper three-port T-connector fitting (58 in FIG. 3).

In general, each of the T-connector fittings 57, 58 has a first port (76 in FIG. 4 for the fitting 57), a second port (77 in FIG. 4 for the fitting 57) opposite the first port, and a third port (78 in FIG. 4 for the fitting 57), and the inner tube 72 passes through the first port of each of the T-connector fittings and the outer tube 71 is attached to the first port of each of the T-connector fittings, and in each of the T-connector fittings, a first gas flow passage 73 outside of the inner tube 72 and inside the outer tube 71 extends through the first port to the third port, and a second gas flow passage 74 inside the inner tube 72 extends to the second port.

As shown in FIG. 4, the outer tube 71 is attached externally to the arm 76, for example, by a weld, a brazing alloy seam, or a solder seam 81. The inner tube 72 is attached internally to the arm 77, for example by a weld, a brazing alloy seam, or a solder seam 82. The outer tube 71 and the inner tube 72 are attached in a similar fashion to the upper three-port T-fitting (58 in FIG. 3). The three-port T-fittings 57, 58 can be made from SWAGELOK® brand T-fittings, sold by Swagelok Company of Solon, Ohio, such as ¼ inch union tee fittings, part No. SS-400-3 or SS-4-VCR-T. In this case, for each fitting, the two ends of the fitting not attached to the outer tube 71 can be connected to the other components of the gas circulation loop using standard screw-on tube connectors or metal gasket fittings. These two ends of each fitting could also be welded, braised, or soldered to the other components of the gas circulation loop, or these two ends of each fitting could be provided with custom terminations for connection to the other components of the gas circulation loop.

The three-port T-fittings 57, 58 can be attached to the outer and inner tubes 71, 72 of the tubular section 56 either before or after winding of the tubular section 56 around a cylindrical grooved bender die to form the helix. Attachment of the three-port fittings 57, 58 to the tubular section 56 before winding of the tubular section 56 into a helix may result in a more concentric relationship between the outer tube 71 and the inner tube 72.

A working example of the cryogenic system of FIG. 2 was constructed for measuring the critical current of a superconducting wire sample (i.e., the material 43 to be cooled to a cryogenic temperature). The housing 41 was about 25 centimeters in height, 30 centimeters in width, and 20 centimeters in depth. The components in the internal cryogenic vacuum chamber 42 were wrapped with super-insulation. The sample was about four centimetres in length. The sample and lead wires to the sample were cooled by direct contact with a flow of helium gas through the canister 44 and circulating in the circulation loop 49.

The two-stage cryocooler 45 was a model SHI CH204 10K cryocooler sold by Sumitomo (SHI) Cryogenics of America, Inc., of Allentown, Pa. The model SHI CH204 cryocooler should have a base temperature at the second stage cold head 47 with no load of about 9-10 K, and a cooling capacity of about 7 watts at 20 K. The first-stage heat exchanger 50 had a helical path about the first stage cold head 46 while the second-stage heat exchanger 51 had a serpentine path under the second stage cold head 47.

The circulation pump 55 was a room-temperature diaphragm pump, model KNF NO22AN.18, sold by KNF Neuberger, Inc., of Trenton, N.J. The circulation loop 49 was vacuum purged and then charged with helium gas at about 0.3 bar over atmospheric pressure. The helium gas pressure differential across the circulation pump 55 was about 0.1-0.2 bar (1.5-3 psi), at a flow rate of about 10-15 liters per minute.

FIG. 5 shows temperatures measured at various locations in the working example of the system 40 of FIG. 2 during cooling of the cryogenic system 40 from an ambient room temperature of 295 degrees Kelvin. The two-stage cryocooler 45 and the gas pump 55 were turned on at the time of zero minutes. During the measurements of FIG. 5, a sample 43 was absent from the canister 44.

The four upper curves 90, 91, 92, 93 in FIG. 5 show the temperature as a function of time at the inlets and outlets of the counter-flow heat exchanger 54. The top curve 90 shows the inlet temperature at the upper fitting 58 of the flow of helium gas from the gas pump 55. The curve 91 shows the outlet temperature at the upper fitting 58 of the flow of helium gas to the gas pump 55. The curve 92 shows the outlet temperature at the lower fitting 57 of the flow of helium gas to the first-stage cold head 46. The curve 93 shows the inlet temperature at the lower fitting 57 of the flow of helium gas from the canister 44. The curve 94 shows the temperature of the flow of helium gas at the lower port 52 of the canister 44. The curve 95 shows the temperature of the first stage cold head 46 of the two-stage cryocooler 45. The curve 96 shows the temperature of the second stage cold head 47.

The temperature measurements of FIG. 5 indicated that the hot end of the counter-flow heat exchanger 54 (i.e., the outlet flow from the upper fitting 58) remained within 5 K of ambient temperature, with a temperature difference of less than 5 K between inlet flow and outlet flow of the upper fitting 58. The cold end of the counter-flow heat exchanger 54 (i.e., the inlet flow into the lower fitting 57) reached a temperature less than 40 K, with a temperature difference of less than 50 K between the inlet flow and outlet flow of the lower fitting 57. In this example the counter-flow heat exchanger 54 operated at over 85% efficiency.

The various embodiments described above are provided by way of illustration only and should not be construed to limit the scope of the disclosure. Therefore, many such details are neither shown nor described. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of shape, size and arrangement of the parts within the principles of the present disclosure to the full extent indicated by the broad general meaning of the terms used in the attached claims. It will therefore be appreciated that the embodiments described above may be modified within the scope of the appended claims. Claim language reciting “at least one of” a set indicates that one member of the set or multiple members of the set satisfy the claim. 

What is claimed is:
 1. A cryogenic system comprising: a housing of a cryogenic chamber; a cold source in the cryogenic chamber; and a gas circulation loop for circulating cryogenic gas between the cold source and material to be cooled in the cryogenic chamber; wherein the gas circulation loop includes a gas pump, and a counter-flow heat exchanger connecting the gas pump to the cold source for cooling an inlet flow of the cryogenic gas from the gas pump to the cold source with an outlet flow of the cryogenic gas from the material to be cooled in the cryogenic chamber to the gas pump.
 2. The cryogenic system as claimed in claim 1, wherein the gas pump is located outside of the cryogenic chamber.
 3. The cryogenic system as claimed in claim 1, which includes a cryocooler having a cold head in the cryogenic chamber, the cold source includes the cold head, and the gas circulation loop includes another heat exchanger, which is fastened to the cold head and coupled to the counter-flow heat exchanger for receiving the inlet flow of the cryogenic gas from the counter-flow heat exchanger.
 4. The cryogenic system as claimed in claim 1, which includes at least one cryocooler having a plurality of stages, the at least one cryocooler has a plurality of cold heads in the cryogenic chamber, the cold source includes at least two of the cold heads, and the counter-flow heat exchanger is coupled to the cold heads for progressively cooling the inlet flow of cryogenic gas from the counter-flow heat exchanger by heat transfer to a higher-temperature one of the cold heads and then to a lower-temperature one of the cold heads before passing to the material to be cooled in the cryogenic chamber.
 5. The cryogenic system as claimed in claim 1, wherein the counter-flow heat exchanger includes an outer tube and an inner tube nested within the outer tube, and a pair of three-port connector fittings attached to respective ends of the tubes so that the three-port connector fittings provide a sealed environment with independent access to each of the in-flow and out-flow of the cryogenic gas through the counter-flow heat exchanger while preventing mixing of the in-flow and out-flow of the cryogenic gas through the counter-flow heat exchanger.
 6. The cryogenic system as claimed in claim 5, wherein the outer tube is a stainless steel tube, and the inner tube is a copper tube.
 7. The cryogenic system as claimed in claim 5, wherein the tubes are wound into a helix.
 8. The cryogenic system as claimed in claim 5, wherein the three-port connector fittings are T-connector fittings, and each of the T-connector fittings has a first port, a second port opposite the first port, and a third port, and the inner tube passes through the first port of each of the T-connector fittings and the outer tube is attached to the first port of each of the T-connector fittings, and in each of the T-connector fittings, a first gas flow passage outside of the inner tube and inside the outer tube extends through the first port to the third port, and a second gas flow passage inside the inner tube extends to the second port.
 9. A method of cooling a material in a cryogenic chamber, said method comprising circulating cryogenic gas in a gas circulation loop between a cold source in the cryogenic chamber and the material in the cryogenic chamber, wherein the gas circulation loop includes a gas pump and a counter-flow heat exchanger, and said method further includes the gas pump pumping the cryogenic gas through the gas circulation loop, and the counter-flow heat exchanger cooling an inlet flow of the cryogenic gas from the gas pump to the cold source with an outlet flow of the cryogenic gas from the material in the cryogenic chamber to the gas pump.
 10. The method as claimed in claim 9, wherein the gas pump is operating in a room-temperature environment to circulate the cryogenic gas through the gas circulation loop.
 11. The method as claimed in claim 9, wherein the cold source is providing a temperature below 70 degrees Kelvin, and the material in the cryogenic chamber is cooled to below 70 degrees Kelvin.
 12. The method as claimed in claim 9, wherein the cold source is a cold head of a cryocooler, and the gas circulation loop includes another heat exchanger, which is fastened to the cold head and receiving the inlet flow of the cryogenic gas from the counter-flow heat exchanger.
 13. The method as claimed in claim 9, wherein the cold source includes at least two cold heads of at least one cryocooler having the cold heads in the cryogenic chamber, and the inlet flow of cryogenic gas from the counter-flow heat exchanger is progressively cooled by heat transfer to a higher-temperature one of the cold heads and then to a lower-temperature one of the cold heads before passing to the material to be cooled in the cryogenic chamber.
 14. The method as claimed in claim 9, wherein the counter-flow heat exchanger includes an outer tube and an inner tube nested within the outer tube, and a pair of three-port connector fittings attached to respective ends of the tubes so that the three-port connector fittings provide a sealed environment with independent access to each of the in-flow and out-flow of the cryogenic gas through the counter-flow heat exchanger while preventing mixing of the in-flow and out-flow of the cryogenic gas through the counter-flow heat exchanger.
 15. The method as claimed in claim 14, wherein the three-port connector fittings are T-connector fittings, and each of the T-connector fittings has a first port, a second port opposite the first port, and a third port, and the inner tube passes through the first port of each of the T-connector fittings and the outer tube is attached to the first port of each of the T-connector fittings, and in each of the T-connector fittings, a first gas flow passage outside of the inner tube and inside the outer tube extends through the first port to the third port, and a second gas flow passage inside the inner tube extends to the second port.
 16. The method as claimed in claim 15, wherein the in-flow of cryogenic gas through the counter-flow heat exchanger passes through the first gas flow passage, and the out-flow of cryogenic gas through the counter-flow heat exchanger passes through the second gas flow passage.
 17. A counter-flow heat exchanger comprising an outer tube and an inner tube nested within the outer tube, and a pair of three-port connector fittings attached to respective ends of the tubes so that the three-port connector fittings provide a sealed environment with independent access to a first flow of fluid and a second flow of fluid while preventing mixing of the first flow of fluid and the second flow of fluid, wherein the first flow of fluid flows between the three-port connector fittings inside the outer tube and outside the inner tube, and the second flow of fluid flows between the three-port connector fittings inside the inner tube.
 18. The counter-flow heat exchanger as claimed in claim 17, wherein the outer tube is a stainless steel tube, and the inner tube is a copper tube.
 19. The counter-flow heat exchanger as claimed in claim 17, wherein the tubes are wound into a helix.
 20. The counter-flow heat exchanger as claimed in claim 17, wherein the three-port connector fittings are T-connector fittings, and each of the T-connector fittings has a first port, a second port opposite the first port, and a third port, and the inner tube passes through the first port of each of the T-connector fittings and the outer tube is attached to the first port of each of the T-connector fittings, and in each of the T-connector fittings, a first gas flow passage outside of the inner tube and inside the outer tube extends through the first port to the third port, and a second gas flow passage inside the inner tube extends to the second port. 